The present invention relates to units that can freewheel and, in particular, to units for transmitting torque in one sense, but yet allowing a freewheeling in the event that the output speed tends to exceed the input speed.
In the helicopter art it is known to drive a helicopter blade through a freewheel unit that includes a sprag clutch. Known sprag clutches include a plurality of peripheral cam-like members. Rotation in one direction tends to drive the cam-like members into a tangential position. Rotation in the opposite direction tends to align the cam-like members radially, thereby locking inside and outside hubs to allow torque transmission.
It is also known to transmit torque in a single direction by means of a well-known ratchet. Other existing unidirectional devices include a roller clutch having a plurality of cylindrical rollers disposed around the periphery of the clutch. Each roller is located on a ramp and is spring biased on the ramp. Rotation in one direction tends to drive the roller along the ramp so that interference does not occur. However, rotation in the opposite direction tends to cause the roller to ride up the ramp so that coupling occurs between the inside and outside hub, thereby allowing torque transmission.
Another known unidirectional torque coupler is a spring wrap clutch. This clutch is essentially a spring-like member which can be wound around or inside two hollow, spaced, coaxial members. When the driving member is spinning in a direction to "unwrap" the spring clutch, substantially no torque is transmitted. Rotation of the driving member in the opposite direction tends to wind the spring tightly about the driving and driven members thereby causing torque transmission.
A disadvantage with the foregoing types of freewheeling units is that they inherently rely on a frictional effect or the high vibrational ratcheting effect to cause selective torque transmission. Accordingly, there is a tendency for high wear.
Accordingly, there is a need for a freewheel unit which can allow torque transmission in one direction and also provide positive coupling in a small and reliable package.